The present invention relates to an electrically programmable and erasable memory device having a floating gate electrode (hereinafter abbreviated as EEPROM).
Widely used EEPROMs are relied upon Fowler-Nordheim tunneling phenomenon, in which carriers such as electrons are injected into the floating gate electrode from the drain region and injected from the floating gate electrode back to the drain region through a thin insulating film provided on a part of the drain region. This technology is disclosed in U.S. Pat. No. 4,203,158. In the programming operation, that is, writing operation, the P-type substrate and the control gate electrode are supplied with a , reference potential, and the N-type source region maintains floating state or is applied with low positive potential such as +5 volts. High positive potential such as +20 volts are applied to the N-type drain region so that a high electric field is produced in the thin insulating film made of silicon oxide on the part of the drain region by capacitive coupling and electrons are injected from the floating gate electrode to the drain region through the thin insulating film by Fowler-Nordheim tunneling mechanism. Ccnsequently, positive carriers are accumulated in the floating gate electrode. In the erasing operation, for example, the semiconductor substrate, the source region and the drain region are made to take ground potential, and by applying a high positive potential such as +20 volts to the control gate electrode a high electric field, which is the opposite direction from the writing operation mentioned above, is produced in the thin insulating film and electrons are emitted from the drain region to the floating gate electrode through the thin insulating film by Fowler-Nordhein tunneling so that negative carriers, that is, electrons are accumulated in the floating gate electrode. The writing operation and the erasing operation mentioned above can be replaced with each other. In the reading operation, an adequate voltage is applied to the region between the source and drain regions of the EEPROM is ON state or OFF state, as done in usual nonvolatile memory cells. Therefore, in order to read data correctly in the reading operation, the floating gate electrode must store a sufficient amount of positive or negative carriers so as to set the threshold voltage of the programmed EEPROM to a predetermined level.
In EEPROM, the writing and erasing operations are repeatedly carried out, with high electrid fields of opposite polarities being repeatedly applied to the thin insulating film. Therefore, the thin insulating film is deteriorated. In this case, Fowler-Nordheim tunneling flowing of carriers cannot be desirably conducted under the normal operating conditions, and once stored carriers in the floating gate electrode are apt to leak through the deteriorated thin insulating film. Then, sufficient amount of carriers cannot be stored and the threshold voltage cannot be set at a predetermined value. Consequently, the correct reading cannot be expected
The fatigue phenomenon of the thin insulating film by repeated application of alternative high electric fields is related to electron traps dispersed in the thin insulating film. The formation of the electron traps is due to undesirable minute particles or dusts produced in the process step of forming the thin insulating film. However, this drawback has been recently avoided by introducing super-clean process technology. In the recent EEPROM, the fatigue phenomenon of the thin insulating film does not happen till the repeated applications reach about 10.sup.5 times. A stable EEPROM which can withstand such an enormous repetition of voltage application is required.